Various systems are available and used to manufacture sheets of paper and other paper products. The sheets of paper being manufactured often have multiple characteristics that are monitored and controlled during the manufacturing process, such as dry weight, moisture, and caliper (thickness). The control of these or other sheet properties in a sheet-making machine is typically concerned with keeping the sheet properties as close as possible to target or desired values.
There are often two different types of actuators that are used to control the sheet properties in a sheet-making machine. First, there are machine direction (MD) actuators that typically affect only the cross direction average of a sheet property. The MD actuators often can have different dynamic responses with respect to a sheet property. Second, there are cross direction (CD) actuators that are typically arrayed across a sheet in the cross direction. Each array of CD actuators can usually affect both the average of a sheet property and the cross direction shape of the sheet property. The CD actuators often can have different dynamic responses and different spatial responses with respect to a sheet property.
The sheet properties are typically measured by a set of sensors scanned across the sheet in the CD direction by a scanner. A sheet property may also be measured by a stationary array of sensors. Many sheet-making machines have multiple scanners at different locations, and each scanner typically has its own set of sensors. The scanners are often not synchronized and can have very different scan times. In conventional CD control systems, synchronization of the sensors was not necessary since each CD controller was only concerned with controlling one sheet property measured by one set of sensors. The conventional CD controllers typically ignored the fact that one CD actuator could have an effect on multiple sheet properties. However, for multivariable CD controllers that implement coordinated control of multiple sheet properties, it is often required that the measurements of the sheet properties be synchronized.